Selective organic matter preservation in "burn-down" turbidites on the Madeira Abyssal Plain

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  • Oxidized intervals of five organic‐rich Madeira Abyssal Plain (MAP) turbidites deposited during the Miocene, Pliocene, and Pleistocene all displayed comparable major loss of total organic carbon (TOC) (84 ± 3.1%) accompanied by a negative isotopic (δ¹³C) shift ranging from −0.3 to −2.9‰. Major but significantly lower loss of total nitrogen (Ntot, 61 ± 7.1%) also occurred, leading to a decrease in TOC relative to Ntot (C/Ntot) and a +1.3 to 2.7‰ Ntot isotopic (δ¹⁵N) shift. Compound specific isotopic measurements on plant wax n‐alkanes indicate the terrestrial organic component in the unoxidized deposits is ¹³C‐enriched owing to significant C4 contribution. Selective preservation of terrestrial relative to marine organic carbon could account for the δ¹³C behavior of TOC upon oxidation but only if a ¹³C‐depleted component of the bulk terrestrial signal is selectively preserved in the process. Although the C/Ntot decrease and positive δ¹⁵N shift seems inconsistent with selective terrestrial organic preservation, results from analysis of a Modern eolian dust sample collected in the vicinity indicate these observations are compatible. Regardless of the specific explanation for these isotopic observations, however, our findings provide evidence that paleoreconstruction of properties such as pCO₂ using the δ¹³C of TOC is a goal fraught with uncertainty whether or not the marine sedimentary record considered is “contaminated” with significant terrestrial input. Nonetheless, despite major and selective loss of both marine and terrestrial components as a consequence of postdepositional oxidation, intensive organic geochemical proxies such as the alkenone unsaturation index, U₃₇ᴷ', appear resistant to change and thereby retain their paleoceanographic promise.
  • Keywords: Organic carbon, Diagenesis, Turbidites, Stable carbon isotopes, Lipid biomarkers
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  • Prahl, F. G., Cowie, G. L., De Lange, G. J., & Sparrow, M. A. (2003). Selective organic matter preservation in "burn-down" turbidites on the Madeira Abyssal Plain. Paleoceanography, 18(2). doi:10.1029/2002PA000853
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  • 18
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  • NSF grant OCE-961685 (FGP).



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