A case of post-depositional aerobic degradation of terrestrial organic matter in turbidite deposits from the Madeira Abyssal Plain

A set of oxidized and unoxidized sediment intervals from the f-turbidite identified in two piston cores from the Madeira Abyssal Plain (MAP) was analyzed for calcium carbonate, total organic carbon (TOC), total nitrogen (TN), stable carbon composition of TOC (δ13C ), total hydrolyzable amino acids (THAA), total neutral sugars (TSUG), lignin phenols (LIG), two different lipid biomarkers of marine phytoplankton origin and vascular plantwax n-alkanes and n-acids. Comparison shows changes for all properties depicting the effect of post-depositional, aerobic oxidation. The oxidation process re-mineralized approximately 80% of the TOC and approximately 60% of the TN in the original deposit and left the residual organic matter depleted in 13C by 1.7–2.9‰ THAA and TSUG account for 10% or less of the TOC in all samples leaving approximately 90% of the TOC chemically unidentified. THAA, TSUG, LIG, marine phytoplankton biomarkers and plantwax n-acids were lost from the oxidized deposit to an extent as great as that for TOC. Two non-protein amino acids (β-alanine and γ-aminobutyric acid) and plant wax n-alkanes displayed much less degradation (9–37%) and concentrated 3.3–4.8 times relative to TOC by the oxidation process. These observations show that both marine and terrestrial components contribute to the re-mineralized TOC fraction. A binary mixing approach was used to model quantitatively the change in δ13C and plantwax n-alkane concentration. Results suggest that terrestrial organic carbon contributes approximately 15% to the TOC content of the original turbidite, its abundance increases approximately 2–4 times as a consequence of the oxidation process and approximately 40 and 90% of the terrestrial and marine component of TOC in the original turbidite, respectively, was destroyed by the oxidation process. Although selective preservation of terrestrial relative to marine organic carbon is a well-documented phenomenon in sedimentary processes, this study represents the first attempt to assess the sensitivity of terrestrial organic matter to oxidative degradation in a sedimentary environment.