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Multiple drivers of Miocene C4 ecosystem expansions

Nature Geoscience volume 13, pages 463–464 (2020)Cite this article

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Matters Arising to this article was published on 02 July 2020

The Original Article was published on 22 July 2019

arising from P. J. Polissar et al. Nature Geoscience https://doi.org/10.1038/s41561-019-0399-2 (2019)

Plant-wax derived n-alkanes consist of a series of straight-chain, odd-carbon-numbered saturated hydrocarbons, of which the n-C29 and/or n-C31 alkanes are usually the most abundant. The n-C35 alkane analysed by Polissar et al.3 is a minor component in most plants4,5 and usually of very low abundance in sediments6. Its low abundance in sediments makes it difficult to analyse its δ13C compositions due to background effects and potential coelution with other compounds. We are not aware of other studies using the δ13C of the n-C35 alkane as specific marker for C4 plants as advocated by Polissar and colleagues3. With the sample preparation and analytical procedures used by Polissar et al.3 there will, for instance, be potential co-elution of lycopane with the n-C35 alkane in marine sediment samples7. Lycopane, a photoautotrophic produced compound8, has elevated δ13C signatures due to dissolved inorganic carbon uptake7 and will result in elevated δ13C signals if its presence cannot be completely ruled out. Some plant studies report the elevated presence of n-C35 alkane in graminoids4, which forms the basis of the argument put forward by Polissar and co-authors3. If the n-C35 alkane were derived predominantly from C4 plants, we would expect its relative abundance to rise after 10 Myr ago (Ma), which is not the case (Fig. 1b). If C3 plants were replaced by C4 plants with similar n-alkane distributions, the reason that the δ13C response was only strong in the n-C35 alkane and subdued for the more abundant n-C29 to n-C33 alkanes is unclear (Fig. 1d). Small δ13C changes can be induced by other effects such as ecosystem-specific water-use efficiency and ecosystem structure, and do not necessarily indicate C4 plant contributions5. The n-C35 alkane thus apparently recorded a different δ13C signal than the other long-chain n-alkanes. Based on the above-mentioned considerations, we see no evidence that the contribution of C4 plants to the sediments increased substantially before 8–7 Ma and advise caution in interpreting low-abundance compounds with yet unresolved importance.

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Fig. 1: Drivers of Neogene grassland expansion at ODP Site 659.

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  1. MARUM – Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany

    Enno Schefuß & Lydie M. Dupont

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Schefuß, E., Dupont, L.M. Multiple drivers of Miocene C4 ecosystem expansions. Nat. Geosci. 13, 463–464 (2020). https://doi.org/10.1038/s41561-020-0590-5

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